1. Field of the Invention
The present invention relates to a technique by which a display panel which is flexible (having a flexibility) is provided, and more particularly to a technique by which a flexible active matrix liquid-crystal display unit is provided.
2. Description of the Related Art
There has been known a liquid-crystal display unit as a display unit which is small-sized, light in weight and of the thin type. This has a structure in which liquid crystal is interposed between a pair of translucent substrates which are bonded to each other at intervals of several μm and held in this state as the structure of a display panel. In the operation of the display unit, an electric field is applied to liquid crystal in a predetermined region so as to change its optical characteristics, whereby the presence/absence of a light transmitted through a panel and the amount of transmitted light are controlled.
As a technique by which the display characteristics of this liquid-crystal display unit is further enhanced, there has been known the active matrix display panel. This is to arrange switching thin-film transistors (in general, an amorphous silicon thin film is used) in the respective pixels disposed in the form of a matrix, and to control charges that takes in or out of the respective pixels by the thin-film transistors.
In order to improve the characteristics of the active matrix liquid-crystal display device, it is necessary to improve the characteristics of the thin-film transistor as used. However, under the existing circumstance, it is difficult to improve such characteristics in view of the relationship of the substrate as used.
What is required for the substrate used in the liquid-crystal display panel is such an optical characteristic that the substrate transmits a visible light. Substrates having such an optical characteristic are of a variety of resin substrates, a glass substrate, a quartz substrate, etc. Of them, the resin substrate is low in a heat-resistance, and therefore it is hard to manufacture the thin-film transistor on its surface. Also, the quartz substrate can withstand a high temperature of 1000° C. or more, however, it is expensive and causes an economical problem when the display unit is enlarged. For that reason, the glass substrate is generally used.
In order to improve the characteristics of the thin-film transistor, a silicon semiconductor thin film having a crystalline property need be used for the thin-film semiconductor that forms the thin-film transistor. However, in order to form the crystalline silicon thin film, a sample must be exposed to a high-temperature atmosphere, and in the case of using the glass substrate, there arises such a problem that the substrate is warped or deformed. In particular, when making the substrate large in area, that problem becomes remarkable.
As a technique by which a liquid-crystal display panel that solves such a problem and has a high display characteristic is obtained, there has been known a technique disclosed in Japanese Patent Unexamined Publication No. Hei 6-504139. This technique is that a thin-film transistor is manufactured by using a monocrystal silicon thin film formed through the SOI technique, etc., that thin-film transistor is peeled off from the substrate for an epitaxial growth, and the thin-film transistor is bonded to an arbitrary substrate having an optical characteristic as required, to thereby obtain a panel constituting a liquid-crystal display unit.
In the case of using this technique, since the monocrystal silicon thin film formed using a known SOI technique can be used, a thin-film transistor having a high characteristic can be obtained. Also, a substrate having a curved surface can be used.
In the technique disclosed in Japanese Patent Unexamined Publication No. Hei 6-504139, a thin-film transistor is manufactured using the SOI technique. However, in the SOI technique under the existing circumstance, it is difficult to form a monocrystal thin film in a large area of 10 inch diagonal or more.
For example, under the existing circumstance, the maximum monocrystal wafer is of 16 inches in size. In this case, the maximum square panel as obtained is of 16×(½)−2=11 inch diagonal.
On the other hand, it is expected that the liquid-crystal display panel as required is of 20 or 30 inches or more in the diagonal dimension in the future. It is impossible to constitute such a large-sized liquid-crystal display panel through the method using the known SOI technique.